Spiders and other arachnids have knees — though no kneecaps. Their knees, like ours, connect longer leg parts, but spider knees can move much more freely.
What good are knees to a spider? After all, other arthropods — insects, lobsters, centipedes, and so forth — get along without them. All have segmented legs, but only arachnids — the subgroup that includes spiders, scorpions, and mites — have knees. The arachnid knee, or patella, is short and bell-shaped. Interspersed between longer segments, the knee increases overall limb flexibility. The arachnid knee makes it possible for the leg below it to move independently in almost any direction.

Dachshunds and Spider Knees

While trying to learn why some spiders have longer legs than others, scientists from Germany's University of Göttingen were surprised to discover that spiders have not one but two dachshund genes. The dachshund gene was named after the stumpy-legged wiener dog bred to poke its long body into badger holes. The dachshund (dac) gene was originally discovered in fruit flies, which have only one, and dac-mutant flies have short, malformed legs. The spider's dachshund genes don't affect the length of its legs, but one of them, it turns out, gives the spider its knees!